Zeolite Omega was first synthesized more than fifteen years ago. The synthesis techniques and characterization of the synthetic zeolite are reported in U.S. Pat. No. 4,241,036 issued Dec. 23, 1980, to E. M. Flanigen et al, the entire disclosure of which is incorporated by reference herein. Other synthesis processes have subsequently been developed in which the organic templating agent employed is a different organic amine, namely, (.beta.-hydroxyethyl) trimethylammonium hydroxide (choline), choline chloride, pyrrolidone, or 1,4-diazobicyclo (2.2.2) octane (DABCO), the latter also called triethylenediamine (TED). These processes are disclosed, respectively, in U.S. Pat. Nos. 4,021,447 and 4,331,643, both issued to Rubin et al. See also U.S. Pat. No. 4,377,502 for additional synthesis procedures. Zeolite Omega is typically crystallized hydrothermally from a reaction mixture having a composition expressed in terms of mole ratios of oxides within the ranges
______________________________________ (Na.sub.2 O + R.sub.2 O)/SiO.sub.2 from about 0.1 to about 0.6 R.sub.2 O/(R.sub.2 O + Na.sub.2 O) from &gt;0 to about 0.6 SiO.sub.2 /Al.sub.2 O.sub.3 from about 5 to about 30 H.sub.2 O(R.sub.2 O + Na.sub.2 O) from about 10 to about 125 ______________________________________
wherein "R" represents the tetramethylammonium or other organic cation. Crystallization periods of from about 1 to 8 days at temperatures of from 90.degree. C. to 180.degree. C. are usually satisfactory. The as-synthesized zeolite Omega typically has a chemical composition (anhydrous basis) in terms of molar oxide ratios of EQU (xR.sub.2 O+yNa.sub.2 O):Al.sub.2 O.sub.3 :5to 20 SiO.sub.2
wherein (x+y) has a value of from about 1.0 to 1.5 and x/y having a value usually in the range of about 0.35 to 0.60.
In addition to composition and in conjunction therewith, zeolite .OMEGA. can be identified and distinguished from other crystalline substances by its X-ray powder diffraction pattern, the data for which are set forth below in Table A. In obtaining the X-ray powder diffraction pattern, standard techniques were employed. The radiation was the K.sub.60 doublet of copper, and a Geiger counter spectrometer with a strip chart pen recorder was used. The peak heights, I, and the positions as a function of 2.THETA., where .THETA. is the Bragg angle, were read from the spectrometer chart. From these, the relative intensities, and d(.ANG.) observed, the interplanar pacing in Angstrom units corresponding to the recorded lines were determined. In Table A, the more significant interplanar spacings, i.e., the d(.ANG.) values which characterize and distinguish zeolite .OMEGA. from other zeolite species and which must be present in the X-ray powder diffraction pattern of the zeolite .OMEGA. composition of the present invention, are set forth. The relative intensities of the lines are expressed as VS (very strong), S (strong), MS (medium strong) and M (medium).
TABLE A ______________________________________ d, (A) Relative Intensity ______________________________________ 9.1 .+-. 0.2 VS 7.9 .+-. 0.2 M 6.9 .+-. 0.2 M-S 5.95 .+-. 0.1 M-S 4.69 .+-. 0.1 M-S 3.79 .+-. 0.1 S 3.62 .+-. 0.05 M-S 3.51 .+-. 0.05 M-S 3.14 .+-. 0.05 M-S 3.08 .+-. 0.05 M 3.03 .+-. 0.05 M 2.92 .+-. 0.05 M-S ______________________________________
Thus, zeolite .OMEGA. can be defined as a synthetic cyrstalline aluminosilicate having an X-ray powder diffraction pattern characterized by at least those interplanar spacing values set forth in Table A and having the stoichiometric compositions as set forth hereinbefore. The X-ray data given below in Table B are for a typical example of zeolite .OMEGA. prepared in the sodium, TMA system.
TABLE B ______________________________________ d, (A) Intensity ______________________________________ 15.95 20 9.09 86 7.87 21 6.86 27 5.94 32 5.47 6 5.25 * 8 5.19 4.695 32 3.909 11 3.794 58 3.708 30 3.620 25 3.516 53 3.456 20 3.13 38 3.074 * 21 3.02 2.911 36 2.640 6 2.488 6 2.342 17 2.272 6 2.139 5 2.031 17 1.978 5 1.909 10 1.748 6 ______________________________________ * = doublet
As synthesized, the crystallites of zeolite Omega are usually quite small and are recovered as anhedral to spherical growth agglomerates from about 0.2 to several microns in size. The acid stability of the crystals is relatively high, producing a buffering effect at a value of about 1.6 when titrated with a 0.25N aqueous solution of HCl. By this test zeolite Omega is more acid-stable than zeolite Y, less acid stable than mordenite, and essentially the same as the natural zeolites erionite, clinoptilolite and chabazite.
The pore diameters of the zeolite are quite large, at least about 8 Angstroms, as evidenced by the adsorption of more than 15 weight percent of (C.sub.4 F.sub.9).sub.3 N at 50.degree. C. and a pressure of 0.7 mm. Hg in each of the sodium, calcium, potassium and ammonium cation forms after calcination to remove the organic ions and/or compounds from the internal cavities. The organic species are not capable of being removed as such through the pore system, because they are intercalated in the structural gmelinite cages which are arranged in the crystal lattice to form the large pores along the crystallographic "c" axis.
The basic chemical and physical properties of zeolite Omega as indicated by the aforementioned evaluations suggest, a priori, that it would have considerable commercial capabilities as a catalyst or catalyst base in many of the hydrocarbon conversion processes which employ other large-pore zeolites such as zeolite Y. This potential has not been realized, however, due in large part to an apparent lack of consistency in the acidic and adsorptive properties observed in different synthesis batches of the zeolite.
A particularly important and significantly variable property is the thermal stability of the as-synthesized zeolite Omega. As reported by Weeks et al in JCS Farad. Trans. 1, 72 (1976), zeolite Omega in the sodium or ammonium cation form is either destroyed or undergoes a considerable decrease in crystallinity by calcination in air at 600.degree. C., a phenomenon attributed by the authors to the loss of TMA.sup.+ ions at about that temperature. Others have suggested that the small crystallite size of the analyzed samples is, in part at least, responsible for the relatively low thermal stability. On the other hand, in U.S. Pat. No. 4,241,036, Flanigen et al report that zeolite Omega is stable up to about 800.degree. C. when heated in air or vacuum and that when heated for 17 hours at temperatures within the range of 300.degree. C. to 750.degree. C., the zeolite undergoes no appreciable loss in X-ray crystallinity, but that at 400.degree. C. there is an appreciable loss of TMA.sup.+ cations by thermal decomposition. It would appear from the foregoing that one or more aspects of the synthesis procedure, as yet unidentified, can have a marked effect upon the physical and/or chemical properties of zeolite .OMEGA., and that these differences account, at least in part, for the somewhat erratic catalytic properties noted by prior investigators in zeolite .OMEGA. compositions which, by virtue of their provenance, would be expected to be nearly identical.
In U.S. Pat. No. 4,780,436, issued Oct. 25, 1988, to Raatz et al, this characteristic instability of zeolite .OMEGA. is discussed and a stabilization and dealumination procedure is proposed which converts the as-synthesized form of the zeolite to a form with more reproducible catalytic behavior. The treatment proposed by Raatz et al is a three-step procedure which comprises:
(a) a first step of subjecting the synthetic zeolite to a treatment for removing the major part of the TMA.sup.+ cations, while decreasing the alkali metal cations to less than 0.5 percent by weight;
(b) a second step of subjecting the product of the first step to at least one calcination in air, steam or a mixture of air and steam at a temperature of from 400.degree. C. to 900.degree. C.; and
(c) acid etching the product of the second step with an inorganic acid such as HCl or an organic acid such as acetic acid.
The procedure is alleged to cause some shrinkage of the unit cell constants, a.sub.o and c.sub.o to below 1.814A and 0.759A, respectively, to increase the nitrogen adsorption capacity and to create a mesopore structure which corresponds to about 5 to 50 percent of the combined mesopore and micropore volume. The catalytic acidity of the zeolite is also said to be improved.